The electrical resistivity of a subsurface earth formation can be strongly dependent on the direction of the current flow that experiences the resistivity. Often as a result of a finely layered structure of the rocks, the electrical resistivity in the vertical direction may typically be larger, typically by two or more times, than the electrical resistivity in a horizontal direction. This phenomenon is generally referred to as vertical electrical anisotropy. Even where individual sedimentary layers are electrically isotropic, the combined effect of many of such layers together may cause a “macrosopic” anisotropic behaviour.
WO 2006/135510 discloses a marine method for determining the earth vertical electrical anisotropy from an electromagnetic survey of a subsurface region located below the water bottom in an offshore environment. The survey uses an electromagnetic source and a plurality of electromagnetic receivers, with which electromagnetic field data is obtained at a plurality of receiver locations. Horizontal and vertical resistivities are calculated by solving Maxwell's electromagnetic field equations at a position in the subsurface formation, using survey acquisition parameters and the measured electromagnetic field data. A measure of the vertical anisotropy is obtained from the calculated horizontal and vertical resistivities.
A drawback of the method of WO 2006/135510 is that it requires multi-azimuth electromagnetic field data, obtained both online relative to a survey line of electromagnetic source positions and offline with respect to that survey line.
Moreover, the method requires the data to comprise at least two field components, one sensitive predominantly to vertical resistivity and one sensitive predominantly to horizontal resistivity.
Another drawback is that the method requires extensive inversion techniques.